The present invention relates to an apparatus and method for treating liquid and semi-solid organic waste materials prior to final disposal, whereby the wastes are made suitable for disposition in a landfill or the like.
Disposal of organic waste materials resulting particularly from sewage, but also from garbage, certain industrial wastes, and the like, has long been a significant problem. Numerous techniques for handling the wastes have been developed, typically involving placing the wastes in landfills. Such landfills have, however, resulted in pollution and health problems, as well as making the land so used unfit for other purposes. Many of the waste materials are physically or chemically unstable, and thus decompose under the influence of natural factors, with release of objectionable odors, and toxic or biologically harmful products which can enter into the contiguous water table. Moreover, experience has shown that even with the best landfill techniques there can also result insect, rodent and disease problems, and the potential for air pollution through refuse fires. Inevitable decomposition of non-permanent materials results in reduction of the volume of the material used as landfill with resultant subsidence of the filled land.
Thus, a waste treatment process is needed that renders the wastes non-polluting, solid, substantially insoluable in water, and chemically and physically stable. Such a process that meets these requirements is disclosed in U.S. Pat. No. 3,837,872, issued Sept. 24, 1974. The waste is mixed with an alkali metal silicate in the presence of a setting agent. The silicate and setting agent are used in proportions which cause the mixture to undergo consolidation and solidifcation and form a solid mass which is then used as the landfill material.
Of course, suitable apparatus for carrying out this process is required, including a means for mixing the waste with the silicate and setting agent.
One such apparatus is shown in U.S. Pat. No. 3,893,656, issued July 8, 1975. A mobile unit is disclosed for treating liquid waste such as is stored or deposited within a pond or lagoon. The apparatus, which is mounted on the bed of a trailer, includes a storage bin for holding a quantity of the setting agent and a hopper into which the liquid waste is pumped for combination with the setting agent. A shaft having a plurality of paddle-like blades is rotatably mounted in the hopper for mixing the waste and setting agent. The mixture is pumped from the hopper to the disposal site, and the silicate is introduced into the output line, mixed by the pumping of the material.
Such a device is advantageous in situations where the waste is generally well liquified and held in ponds containing moderate quantitites. In the case of sewage, however, as well as in many other situations, a device of this type often will not be usable. Where the sewage waste is processed directly as a sludge from a municipal waste water treatment facility, for example, the device will not have sufficient capacity to handle the quantity of waste material generated.
Various other mixing devices for differing applications are known. For example, in U.S. Pat. No. 4,281,934, issued Aug. 4, 1981, a mixer of the type also known as a pug mill is disclosed. A pair of counter-rotated mixing shafts are mounted in parallel longitudinally along a trough. A plurality of blades are mounted radially to each shaft, staggered by 90.degree. around each shaft whereby the blades of each follow one another in helical succession. The mixer is designed for use with heavy materials, such as construction materials, and as the shafts are rotated, the material is both mixed and advanced along the trough to a discharge end.
Due to the typical characteristics of the organic waste material to be processed, such as sewage, however, either the mobile unit or the simple mixer discussed above for adding the silicate and setting agent to the waste is insufficient. In addition to the solid and liquid components of the waste, a significant amount of stringy and fibrous material is typically present. In known mixers of the type discussed above, the relatively square, radially-extending blades or paddles tend to be coated by such material, reducing the mixer's effectiveness, and requiring halting of the mixer for cleaning. In addition, the high viscosity of the waste material requires more than a mixer for combining the waste with the silicate and setting agent. An entire processing system is required to reduce the waste viscosity to a level that will permit adequate mixing, and allow sufficient throughput to make processing the waste practical.
What is needed, therefore, is an apparatus and method for producing a fertilizer from organic waste material. Such an apparatus and method should be operative to combine the waste material, carried in a sludge, with a silicate and a setting agent in manner that renders production of such a fertilizer both practical and economical.